In general, in electric discharge machining, metal chippings, water, carboxylic acid, and the like, which are caused by electric discharge, degrade a machining liquid and reduces the volume resistivity of machining liquid. When this reduced volume resistivity causes abnormal electric discharge to pass an arc current, there are presented problems of decreasing machining speed and altering the quality of machined surface of an object to be machined.
To avoid this abnormal electric discharge, there is proposed a conventional electric discharge machining method for monitoring voltage across a machining gap and providing a servo reference value of setting a machining gap capable of developing normal electric discharge by servo control when a voltage across a machining gap is diverged from a reference value (for example, refer to patent document 1).
[Patent document 1] JP-A-06-262435 (Pages 2 to 4, FIGS. 1 and 5)
Because the conventional electric discharge machining method is constructed in the manner described above, abnormal electric discharge can be avoided but machining conditions such as discharging time and non-operating time are not optimized, so that sufficient machining characteristics cannot be obtained. In general, it is required that the electric discharge machine satisfies at least three machining characteristics of machining speed, reduced consumption of a machining electrode, and work surface quality. However, the conventional electric discharge machining method is an electric discharge machining method for mainly avoiding work surface quality from being degraded by abnormal electric discharge and hence presents problems that other characteristics (machining speed and reduced consumption of a machining electrode) are not sufficient.
This invention has been made to solve the above problems. The object of the invention is to provide such a method for optimizing machining conditions of an electric discharge machine that can not only avoid work surface quality from being degraded by abnormal electric discharge but also satisfy machining speed and reduced consumption of a machining electrode regardless of the kind and the degree of degradation of a machining liquid.